To answer your question. This technique has a lot of meanings. First of all it has been created to soften prints of extremly contrasty negatives on old graded papers.
For this prupose the paper has to be dipped in developer prior exposure - a great mess of course. After a short exposure the shadows will appear after one or two or more minutes and work as a mask for further exposure(s). After the second or third exposure the print will developed (out) in the tray as usual (or even not).
Depending on the strength of the used developers, the time between first and second exposure, and agitation in the tray (or not!) you can recieve funny results including solarisation, or just the tonal separation you want, independetly from the straight curve of your negative.
For example, if you want deep black shades but for the rest a flatter/softer curve, develop in the easel until the shades will occur, the following second exposure has to be two or three times longer - wait some time, or not and develop in the tray with the same, or a softer developer.
